1. Industrial Field of the Invention
The present invention relates to a method of and an apparatus for forming a plurality of halftone images to reproduce a color image, and more particularly, it relates to a technique to restrain moire.
2. Description of the Related Art
Color prints are usually formed by printing four halftone images with respective four primary color inks of yellow (Y), magenta (M), cyan (C) and black (K), one over another. In printing a plurality of halftone images in such a way, the screen angles, or directions in which halftone dots are arranged, are adjusted to prevent moire. For example, when the screen angle of Y plate is set at 0.degree., that of M plate is set at 15.degree., C. plate at 75.degree., and K plate at 45.degree.. Among the four primary colors, the cyan, magenta and black are strongly related to moire generation because they are deep colors. In the above example of the screen angles, moire generation is restrained by retaining a difference of 30.degree. among the three deep colors.
While tangent 0.degree. and tangent 45.degree. are rational numbers, tangent 15.degree. and tangent 75.degree. are irrational numbers. The halftone-dot formation method which includes a screen angle whose tangent is an irrational number is called Irrational Tangent Method. On the other hand, the halftone-dot formation method in which the tangents of all the screen angles are rational numbers is called Rational Tangent Method. Although the Irrational Tangent Method has been dominant in the past, the Rational Tangent Method is becoming popular these days along with the development of computer technology. This is because the Rational Tangent Method is more suitable to computer processing because a certain square area, which involves a plurality of halftone dots, is repeatedly laid out on an image plane to decide the shape and size of halftone dots.
Since the tangent of screen angles are limited to the rational number in the Rational Tangent Method, it is impossible to set the screen angles for the four primary colors to 0.degree., 15.degree., 75.degree. and 45.degree. as in the above example; therefore, it is usual to set the values close to them. However, it is a fairly difficult operation to set the screen angles and the screen pitch for each halftone image so as to restrain moire because the screen pitch, or a pitch of halftone dots, of each halftone image is also related to moire generation.
A method of restraining moire for the rational Tangent Method is described in Japanese Patent Publication Gazette 60-41343. The method is, however, adoptable under limited conditions only, and it is sometimes difficult to implement.